This application relates to light emitting devices, and in particular to high speed light emitting devices useful for optical switching.
In many areas of electronics, high speed optical switching is necessary. For example, opto-isolators which employ a combination of light emmitting device (LED) and optically coupled light detector are used extensively for electrical isolation in telephone equipment and are gaining increasing importance for use with microprocessors. In many of such applications, high switching speeds are necessary to permit interfacing with high speed logic circuits. High quantum efficiency is also desirable for maximum performance. However, in most cases one of these goals is attained only at the cost of the other. For example, it is desirable to provide a graded bandgap region for light emission in Ga.sub.1-x Al.sub.x As heterojunction devices so that photons are absorbed and re-emitted to increase external quantum efficiency. (See Dawson, "High Efficiency Graded-Band-Gap Ga.sub.1-x Al.sub.x As Light Emitting Diodes", Journal of Applied Physics, Vol. 48, pp. 2485-2492 (June 1977), and U.S. Pat. No. 4,275,404 issued to Cassiday, et al.) This recycling of emitted photons, however, has a tendency to reduce the speed of the device. In addition, use of silicon as an impurity dopant in such structures can further decrease speed. Other proposals have taught various bandgap configurations for reducing absorption losses and/or enhancing injection efficiency. For example, U.S. Pat. No. 4,122,486 issued to Ono, shows a III-V heterostructure device where the n-layer has a significantly greater bandgap than an adjacent p-layer to improve injection efficiency. The p-layer is formed on a p+ substrate. U.S. Pat. No. 4,296,425 issued to Nishizawa also shows in one embodiment a device including, successively, a p+ substrate, a p-layer, and an n-layer, where the p-layer has a graded bandgap and the n-layer has a significantly greater bandgap than the p-layer to minimize absorption in the n-layer and p-layer. Such proposals do not teach how the speed of the device may be enhanced.
On the other hand, Dawson, et al, "Reliable, High Speed LEDs for Short-Haul Optical Data Links", Bell System Technical Journal, Vol. 59, pp. 161-168 (February 1980) shows use of a Ga.sub.1-x Al.sub.x As n-layer with a significantly greater bandgap than an adjacent GaAs p-layer combined with a higher impurity concentration in the p-layer to obtain high switching speeds. However, the efficiency of such a device is lowered significantly when operating at the low currents (approximately 10 milliamps) used for opto-isolators. Further, devices used in opto-isolators cannot take advantage of a d.c. pre-bias, as used in data link applications, which contributes to an increased switching speed.
It is therefore a primary object of the invention to provide a heterojunction light emitting device which combines high speed and high efficiency at low current levels.